In general, an ignition device of the capacitor discharge type for an internal combustion engine includes a capacitor for accumulation of ignition energy which is arranged on a side of a primary winding of an ignition coil, a capacitor charging circuit for charging of the capacitor and a discharge switch which is turned on upon feeding of an ignition signal thereto to cause charges in the capacitor to be discharged to the primary winding of the ignition coil, wherein charges in the ignition energy accumulation capacitor are discharged to the primary winding of the ignition coil to cause a high voltage to be induced across a secondary winding of the ignition coil, resulting in ignition operation of the internal combustion engine being carried out.
Such an ignition device may be constructed in such a manner that a magneto mounted on the internal combustion engine is provided therein with an exciter coil, which acts as an ignition power supply to charge the ignition energy accumulation capacitor. In such construction, a rotation angle period during which the exciter coil generates an output voltage of a positive half cycle and that during which it generates an output voltage of a negative half cycle are defined to be a charge period and a non-charge period, respectively, so that the capacitor may be charged by means of an output voltage of the exciter coil obtained during the charge period.
In recent years, an internal combustion engine is requested to meet requirements such as purification of exhaust gas, an improvement in fuel consumption rate, a decrease in noise, an increase in output and the like. In order to meet the requirements, it is required to use a microcomputer to precisely control an ignition timing of the internal combustion engine in view of control conditions such as an engine speed and the like.
A device for controlling the ignition timing by means of a microcomputer is disclosed in, for example, Japanese Patent Application Laid-Open Publication No. 262366/1989. The device disclosed is adapted to control the ignition timing depending on a degree of opening of a throttle valve. Another device for the same purpose is disclosed in Japanese Patent Application Laid-Open Publication No. 163443/1989, which is constructed so as to control the ignition timing in view of both an engine speed and a degree of opening of a throttle valve.
In the following description, the ignition timing will be indicated by a rotation angle of an output shaft (normally, a crank shaft) of the internal combustion engine.
An ignition device for an internal combustion engine which is adapted to control an ignition timing of the internal combustion engine by means of a microcomputer is constructed so as to realize both an engine speed operation means for operating an engine speed or a rotational speed of the internal combustion engine and an ignition timing operation means for operating the ignition timing in view of the engine speed by means of the microcomputer to feed an ignition circuit with an ignition signal at an rotation angle corresponding to an ignition timing operated by the ignition timing operation means, to thereby carry out ignition operation of the engine. In order to ensure that the ignition device carries but such ignition operation, it is required that coincidence of a rotation angle of the crank shaft with the operated ignition timing is detected at every rotation of the engine.
In view of the foregoing, in the conventional ignition device, a predetermined rotation angle position of the crank shaft of the internal combustion engine is defined to be a reference position, so that an ignition timing of the engine at each engine speed is operated in the form of a length of time required when the engine rotates from the reference position to a rotation angle corresponding to the ignition timing. (Such a length of time will be referred to as "ignition timing measuring time" herein.) The microcomputer functions to set the operated ignition timing measuring time in, for example, a counter (or a timer) every time when the reference position is detected and generate an ignition signal when the counter completes measurement of the time set (or counting of clock pulses providing the ignition timing measuring time).
Thus, in an ignition device for internal combustion engine for controlling an ignition timing of the engine by means of a microcomputer, it is required to detect that a rotation angle position of a crank shaft of the engine coincides with a predetermined reference position. For this purpose, the conventional ignition device is provided with a signal generator (pulser) equipped with a rotor rotated in synchronism with the crank shaft of the engine and a signal generating element arranged opposite to the rotor, resulting in the signal generator generating a pulse for detecting the reference position. Various types of signal generators are known in the art. For example, when an internal combustion engine is mounted thereon with a flywheel, a signal generator is typically used which is constructed in such a manner that a peripheral wall of the flywheel on which a reluctor is formed acts as a rotor and a signal generating element is arranged opposite to the rotor. Also, another signal generator is used for this purpose, which is constructed in such a manner that a reluctor is provided on an outer periphery of a cylindrical boss arranged at a central portion of a flywheel in order to mount the flywheel on an internal combustion engine, to thereby permit the boss to act as a rotor and a signal generating element is arranged inside the flywheel so as to be opposite to the rotor. Further, when the magneto is constructed into a rotor inside-rotation type wherein a magnet rotor is rotated inside a stator, a rotor for signal generation which is separate from the magnet rotor is mounted on a revolving shaft of the engine while being opposite to the signal generating element.
Thus, in the conventional ignition device of the capacitor discharge type for the internal combustion engine constructed as described above, a pulse providing the reference position at which measuring of an ignition timing is started is generated by the signal generator. Unfortunately, this requires to mount the signal generator on the engine, leading to an increase in manufacturing cost. In particular, arrangement of the signal generating element on an outer periphery of the flywheel necessarily causes an increase in outer diameter of the magneto.
It would be considered that a reluctor is arranged on a boss of the flywheel while being rendered opposite to the signal generating element arranged inside the flywheel. However, such arrangement would cause a noise signal to occur due to a magnetic flux generated by a magnetic flied from a magnet of the magnet rotor of the flywheel, so that the noise signal thus produced leads to malfunction of the ignition device. In particular, when an exciter coil for inducing a high voltage necessary to charge the capacitor of the ignition circuit is provided on the magneto of the flywheel, the exciter coil is caused to be increased in size, to thereby substantially fail to provide a space required to arrange the signal generating element inside the flywheel.
It is an object of the present invention to provide an ignition device of the capacitor discharge type for an internal combustion engine which is capable of detecting a reference position at which measuring of an ignition timing of the internal combustion engine is initiated by means of an output of a magneto mounted on the internal combustion engine without any signal generator, to thereby eliminate various problems due to use of the signal generator.